A quartz-based optical waveguide has such a property that the effective refractive index n of a core changes with a temperature change. This thermooptic effect may be expressed by the following equation (1). EQU n=n0+.alpha..DELTA.t (1)
wherein n0 represents the effective refractive index before temperature change; .DELTA.t represents temperature change; and .alpha. represents thermooptic effect. PA1 wherein .DELTA..phi. represents phase difference between two arms and k represents the degree of coupling of the directional coupler. PA1 wherein 1 represents length of heater, .DELTA.n represents difference in effective refractive index between two arms, and .lambda. represents transmission wavelength. PA1 a Mach-Zehnder interferometer circuit comprising two directional couplers and two optical transmission lines for connecting the directional couplers to each other; PA1 elements having Peltier effect provided respectively on the two optical transmission lines; and PA1 energizing means for energizing the elements so that heat is generated from one of the elements with absorption of heat being created in the other element.
A quartz-based optical waveguide switch comprises a Mach-Zehnder interferometer circuit. The Mach-Zehnder interferometer circuit comprises two 3-dB directional couplers and two arms (optical waveguides) for connecting the two directional couplers to each other. Cores serving as the arms are provided in the interior of cladding, and heaters are provided on the top surface of the cladding in its portions corresponding to the respective cores.
According to this optical switch, switching is performed in such a manner that heat is generated from one of the two heaters to create a difference in temperature between the cores, thereby changing the effective refractive index n of one of the cores to shift the phase of light propagating through the core by a half-wavelength, which permits the optical path to be switched. The other heater is used for adjusting the wavelength on the reference side when the adjustment by means of the heater used for the switching is unsatisfactory.
The optical output characteristics of the Mach-Zehnder interferometer circuit 14 may be expressed by the following equations (2) and (3). EQU P1=(1-2k)2 cos 2(.DELTA..phi./2)+sin 2(.DELTA..phi./2) (2) EQU P2=4k(1-k)cos 2(.DELTA..phi./2) (3)
The phase difference between the two arms may be expressed by the formula (4). EQU .DELTA..phi.=2.pi.1.DELTA.n/.lambda. (4)
For example, when k of the 3-dB directional coupler is 0.5 with the heater being turned off (.DELTA..phi.=0), P1 is 0 and P2 is 1. In this case, the light is allowed to advance toward a crossport P2. On the other hand, when the heater is turned on to heat one of the arms so as to give 1.DELTA.n=.lambda./2(.DELTA..phi.=.pi.), P1 is 1 with P2 being 0, performing switching. This permits the light to advance toward a throughport P1.
On the other hand, for example, Japanese Patent Laid-Open No. 75228/1984 discloses one example of the 1.times.2 optical switch.
This optical switch comprises: a substrate made of soda glass; a Y-branched optical waveguide provided on the substrate; and a heat generating section and a heat absorbing section each, comprising dissimilar conductors or semiconductors jointed to each other, having Peltier effect, the heat generating section and the heat absorbing section being provided on both sides of the optical waveguide before the Y-branching point. In this case, the dissimilar conductors having Peltier effect are a thin layer of silver (Ag) and a thin layer of copper (Cu) formed by vapor deposition so as to partially overlap with each other.
The prior art techniques, however, had the following problems.
The first problem is derived from the fact that, in the conventional quartz-based optical waveguide type 2.times.2 optical switch, a difference in temperature between the two arms to perform switching is created by heating both the arms by means of respective heaters to raise the temperature.
For example, when the temperature on the reference side is raised due to a rise in environmental temperature, the arm on the higher temperature side should be further heated with the heater, leading to increased power consumption. Further, when the environmental temperature reaches the maximum service temperature of a module, for example, 65.degree. C., the arm on the higher temperature side should be further heated, leading to a fear of an adverse effect on an adhesive or the like used in mounting.
Thus, in the conventional 2.times.2 optical switch, both the two heaters are heated to create a difference in temperature between the two arms, making it difficult to efficiently conduct switching.
The second problem is that in the conventional Y-shaped 1.times.2 optical switch, the extinction ratio is poor and the crosstalk is large.
The reason for this is that, in the above Y-shaped optical switch, a change in refractive index derived from a temperature change is created within the same core before branching into a Y shape to confine the light within the same core on its side with the refractive index being increased, thereby performing switching. For this reason, some of the light propagating through the core on its side with the refractive index being lowered is radiated toward the cladding.
Further, since the refractive index is changed within the core in its central portion, the temperature control effect of the heat generating section and the heat absorbing section having the Peltier effect interact with each other, making it difficult to create a difference in temperature therebetween. Therefore, at the central portion where the refractive index is created, a possible change in refractive index within the core is merely a broad one, so that the light cannot be confined within the optical waveguide on its one side to such an extent that the light can be confined within the core at the interface of the cladding and the core.
This causes some of the light propagating through the core leaks out toward the cladding and the opposite port, resulting in poor extinction ratio and large crosstalk.
The third problem is that, in the conventional Y-shaped optical switch, the power consumption is large.
This is because, with no current flowing, the above structure functions only as a Y-branched splitter, making it necessary to always supply current for performing switching.
Thus, when use of the structure as a switching device is contemplated, current should be always supplied, leading to large power consumption.
The fourth problem is that the conventional Y-shaped optical switch requires the provision of a circuit for changing the direction of current.
As described above, the reason for this is that, in order to perform switching, the direction of current should be changed and the distribution of the refractive index should be changed symmetrically with respect to the central portion of the optical waveguide.